This invention relates to a demodulator and more particularly relates to a sample data arrangement which enables FM or AM demodulation while the entire arrangement is capable of being implemented in integrated circuit form.
Due to the extensive use of integrated circuits, there has been an increased interest in zero frequency IF receivers. These are radio receivers in which the receive signal is mixed with a local oscillator at its own frequency so as to produce a set of signals at baseband from zero frequency to the modulation frequency band width. The zero IF technique has been described in British Pat. No. 1,530,602 entitled DEMODULATOR FOR FM SIGNALS published on Nov. 1, 1978 to I. A. W. Vance.
In regard to such systems, there has been difficulty with the technique in that the receive side bands which lie both above and below the carrier are "folded" about zero frequency so that they lie one upon the other in the frequency domain. This prohibits the modulation in all but the simplest of cases. In order to overcome this problem, it is necessary to use two local oscillators whose signals are in phase quadrature. By doing this, the system provides two channels in which the signals are out of phase by 90.degree.. The use of two channels enables one to demodulate the desired signal.
As indicated, the above technique has been widely employed in the prior art and such systems require precise phase relationships as well as equal gain in each of the two channels. Based on system operation, the channels have to be balanced in regard to temperature, power and other variations. Thus the zero frequency IF receiver was not widely utilized until the advent of integrated circuits made it possible to provide balance based on common processing techniques and based on the fact that the various components could be integrated on common substrates. In spite of the progress made in integrated circuit technology, it was determined that many of the prior art receivers could not handle a wide variety of signals such as FM signals and tuned circuits were still required.
It is of course obvious that tuned circuits are difficult to implement in integrated circuit form. ln any event, the prior art pursued the zero IF receiver and many techniques were developed which were capable of demodulating several different forms of signals and which were amenable to circuit integration. For example, reference is made to U.S. Pat. No. 3,937,899 entitled TONE DETECTOR USING SPECTRUM PARAMETER ESTIMATION by J. N. Denenberg issued on Feb. 10, 1976. See also U.S. Pat. No. 3,971,988 entitled AM/FM RECEIVER USING SPECTRAL PARAMETER ESTIMATERS issued on July 27, 1976 to J. N. Denenberg.
Apart from the above noted patents, there have been many other patents which relate to the demodulation of AM and FM or AM and PM signals including various forms of FSK as well as some forms of single sideband transmissions.
In reviewing the prior art, it will be seen that the earlier forms of these systems were analog in nature and although certain portions of the systems were capable of being integrated, they were extremely difficult to design and produce. The prior art has been concerned with the replacement of analog circuitry with digital techniques which, as is well known, are easy to integrate. The utilization of digital techniques involves the use of some form of analog to digital converter (A/D) which converts the analog signal to a digital code. The digital signal is then manipulated or processed in order to demodulate the information. Hence such techniques employ extensive look-up tables which are stored in memory and which encompass many bits of storage location.
Pursuant to this approach, the prior art attempted to reduce the storage requirements of memory by utilizing different modulation schemes such as a Sigma-Delta modulator or pulse density modulator which could employ a two-bit coding scheme and hence afford a reduction in the amount of storage necessary. In these systems, the signal was sampled and a value assigned to the phase angle of an FM signal in 45.degree. increments. The output from the system represented the approximate signal and the accuracy was a function of the frequency of sampling. These methods were not capable of simultaneously demodulating an AM/FM signal without further manipulation.
Furthermore, the systems suffer in that they do not provide automatic gain control (AGC) nor could they provide compensation for mistuning or drift in the form of automatic frequency control (AFC). These systems required a set of pulse density modulators and controls as well as a look-up table which therefore resulted in an extremely difficult and complicated arrangement.
It is an object of the present invention to provide a sampling data system for a zero IF receiver which eliminates look-up tables and does not employ analog to digital converters but allows one to directly demodulate an AM or FM signal in a simple and reliable manner and employ circuitry which is capable of being easily integrated.